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arXiv:1503.08252 · Physical Review A

Evaluation of optical probe signals from nonequilibrium systems

Bijay Kumar Agarwalla, Konstantin E. Dorfman, Shaul Mukamel

Published May 4, 2015• 2 citations
arXivHTMLPDF

Abstract

We predict several effects associated with the optical response of systems prepared in a nonequilibrium state by impulsive optical excitations. The linear response depends on the phase of the electric field even if the initial nonequilibrium state has only populations, no coherences. Initial quantum coherences induce additional phase dependence, which also shows new resonances in nonlinear wave mixing. In systems strongly driven by an external optical field, the field frequency generates a phase-dependent probe absorption. This gives further control to manipulate the relative contribution to the linear signal due to initial populations and coherences.

DOI
10.1103/physreva.91.052501

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We predict several effects associated with the optical response of systems prepared in a nonequilibrium state by impulsive optical excitations. The linear response depends on the phase of the electric field even if the initial nonequilibrium state has only populations, no coherences. Initial quantum coherences induce additional phase dependence, which also shows new resonances in nonlinear wave mixing. In systems strongly driven by an external optical field, the field frequency generates a phase-dependent probe absorption. This gives further control to manipulate the relative contribution to the linear signal due to initial populations and coherences.
We explore theoretically the optical response properties in an optomechanical system under an electromagnetically induced transparency condition but with the mechanical resonator being driven by an additional coherent field. In this configuration, more complex quantum coherent and interference phenomena occur. In particular, we find that the probe transmission spectra depend on the total phase of the applied fields. Our study also provides an efficient way to control propagation of a probe field from perfect absorption to remarkable amplification.
We study phase control of linear and nonlinear optical responses in a three-level atomic system in V-configuration exhibiting spontaneously generated coherence (SGC). Because of the SGC effect, the strength of Kerr nonlinearity strongly depends upon the relative phase between the probe and control fields as well as the absorptions. By controlling the relative phase appropriately, large Kerr nonlinearity can be achieved with canceled linear and nonlinear absorptions. Combining with the phase modulation, the strict decay condition of spontaneous decay rates is not needed.
Transient absorption spectroscopy (TAS) conventionally employs a weak probe pulse to passively interrogate pump-induced coherences in a quantum system. Here, we demonstrate a distinct regime in which an intense probe pulse actively controls and interrogates quantum coherences. Using time-dependent perturbation theory, we derive an analytical expression for the absorption cross section that incorporates probe-induced Rabi and Raman effects. Consequently, the system response becomes explicitly probe-intensity dependent, leading to pronounced modifications of both the absorption amplitude and phase. These features are in excellent agreement with density-matrix calculations. We further validate the theory experimentally using TAS of atomic rubidium, where systematic variations of the probe intensity and pump-probe delay directly reveal these predicted features. Our results establish intense probe pulses as a direct and effective tool for manipulating quantum coherences without auxiliary control fields.
A systematic theoretical and computational investigation of the microscopic factors which determine the phase of the signal field in time-resolved quasidegenerate three-pulse scattering experiments is presented. The third-order phase-matched response is obtained by density-matrix perturbation theory using a Green-function formalism for a system composed of two well-separated sets of closely spaced energy levels. Equations for calculating the electric field of four-wave-mixing signals generated by path-length delayed pulses are given. It is found that the phase of the signal field is determined by the excitation pulse phases, the dynamics of the nonlinear polarization decay, the product of four transition dipole matrix elements, and by a pulse-delay-dependent phase modulation at the frequency of the first dipole oscillation in the four-wave-mixing process. Analytic results for a two-level Bloch model show the phase shift from rapid nonlinear polarization decay. The product of dipole matrix elements is real and positive for three-level processes (bleached ground-state absorption and excited-state emission), but can be real and negative for some four-level Raman processes. The pulse-delay-dependent phase modulation treated here is closely related to the interferometric pulse-delay-dependent amplitude modulation observed in some collinear experiments, and plays a role in producing photon echos in inhomogeneously broadened samples. Numerical calculations of phase-resolved electric fields for finite duration pulses using a Brownian oscillator model appropriate for condensed-phase dynamics are presented. The ability of pulse-delay-dependent phase modulation to encode the frequency of the initially excited dipole onto the phase of the signal field can be exploited to examine energy-level connectivity, reveal correlations hidden under the inhomogeneous lineshape, and probe relaxation pathways in multilevel systems.
The response of a coherently prepared four-level λ-type system, interacting with two electromagnetic fields in a Doppler-free pump–probe configuration is analytically formulated. Under density matrix formalism the probe coherence (field-dependent phase) and atomic coherence (field-independent phase) are introduced through the off-diagonal matrix elements. The coherent effects on probe response is investigated for probe coherence, as well as for the driving contribution (coherence) of the pump field. We show how probe coherence can modify the Rabi splitting and two-photon absorption, through shifting and broadening of spectral lines for on- and off-resonance pumping, respectively. In addition, we report on the enhancement of absorptionless dispersion (i.e. refractive index for on and around probe resonance) and two-photon absorption, via coherent control of the driving contribution of the pump field.
We theoretically investigate the response of nonlinear absorption and population dynamics in optically dense media of four-level atoms driven by a single-mode probe laser, via taking the density-dependent near dipole-dipole (NDD) interactions into consideration. The influence of the NDD effects on the absorption of the probe field and population dynamics is predicted via numerical calculations. It is shown that the NDD effects can reduce gradually to transient absorption with the increase of the strengths of the NDD interactions, and transient amplification can be achieved. In the steady-state limit, the probe field exhibits transparency for strong NDD interactions. Alternatively, the population entirely remains at the ground state due to the NDD effects.